performance (rendering): Make Node merge a mutating operation.

We only ever call it in contexts where we don't need the original node
to be untouched.

render/render.go

@@ -180,9 +180,11 @@ func (ret *joinResults) mapChild(from, to string) {
 // Note it is not safe to mix calls to add() with addChild(), addChildAndChildren() or addUnmappedChild()
 func (ret *joinResults) add(from string, m report.Node) {
 	if existing, ok := ret.nodes[m.ID]; ok {
-		m = m.Merge(existing)
+		existing.UnsafeMerge(m)
+		ret.nodes[m.ID] = existing
+	} else {
+		ret.nodes[m.ID] = m
 	}
-	ret.nodes[m.ID] = m
 	ret.mapChild(from, m.ID)
 }